Contact between interconnect and cell in solid oxide cell stacks

ABSTRACT

Improved contact between interconnect and oxygen electrode material in solid oxide cell (SOC) stacks is achieved through a contact point between the oxygen electrode or an oxygen-side contact layer of the SOC and a coated ferritic stainless steel interconnect in the SOC stack, where the coating on the metallic interconnect comprises Cu.

The present invention relates to achievement of improved contact betweeninterconnect and oxygen electrode material in solid oxide cell (SOC)stacks. More specifically, the invention concerns a contact pointbetween an oxygen electrode or an oxygen-side contact layer of a solidoxide cell and a coated ferritic stainless steel interconnect in a solidoxide cell stack.

Solid oxide cells (SOCs) generally include cells designed for differentapplications, such as solid oxide fuel cells (SOFCs) and solid oxideelectrolysis cells (SOECs) which in either case contain a solidelectrolyte layer arranged in between two electrodes, one acting ascathode and the other acting as anode. These types of cells arewell-known in the art and described in i.a. WO 2012/062341 and EP 2 194597 A1, both belonging to the Applicant together with the TechnicalUniversity of Denmark.

A solid oxide fuel cell comprises an oxygen-ion conducting electrolyte,an oxygen electrode (cathode) at which oxygen is reduced and a fuelelectrode (anode) at which fuel (e.g. hydrogen, methane or natural gas)is oxidized. The overall reaction in an SOFC is that the used fuel andoxygen react electrochemically to produce electricity, heat and anoxidized species. The oxidized species is water if hydrogen is used asfuel, carbon dioxide if carbon monoxide is used as fuel, and a mixtureof water and carbon dioxide for hydrocarbon fuels.

A solid oxide electrolysis cell comprises an oxygen-ion conductingelectrolyte, a fuel electrode (cathode) at which an oxidized species(e.g. water or carbon dioxide or both) is reduced with the aid of anexternally applied electric field, and an oxygen electrode (anode) atwhich oxygen ions are oxidized to molecular oxygen. The overall reactionin an SOEC is that the oxidized species are converted electrochemicallyinto reduced species using electricity and heat. If the oxidized speciesfed into the stack is water, hydrogen is formed on the fuel electrode.If the oxidized species is carbon dioxide, carbon monoxide is formed onthe fuel electrode. If the oxidized species is a mixture of water andcarbon dioxide, then a mixture of carbon monoxide and hydrogen (alsoknown as synthesis gas) is produced.

An SOEC operates at temperatures that are suitable for high-temperatureelectrolysis, i.e. temperatures similar to those of an SOFC (from about500 to about 1100° C.). High operating temperatures are needed to ensuresufficiently high oxygen ion conductivity in the electrolyte. Commonlyused electrolyte materials for SOCs include yttria-stabilized zirconia(YSZ), scandia-stabilized zirconia (ScSZ), gadolinia-doped ceria (CGO),samaria-doped ceria (CSO), strontium- and magnesium-doped lanthanumgallates (LSGM), and many others.

SOC electrodes are typically prepared from a composite of anelectronically conductive material and the electrolyte oxide. Forexample, with electrolytes made from YSZ, the conventional fuelelectrode is a Ni—YSZ, ceramic-metallic (cermet) composite. Similarly,oxygen electrodes are typically composites of the electrolyte material(e.g. YSZ or CGO) and oxygen electrode active materials. Oxygenelectrode active materials include perovskites with a general formulaA_(x)BO_(3±δ), where A and B denote metal ions, O denotes oxygen, xindicates the level of A-site non-stoichiometry (excess or deficiency)and δ is indicative of oxygen non-stoichiometry. Examples of relevantperovskites include materials such as strontium-doped lanthanummanganites (LSM), strontium-doped lanthanum ferrites (LSF),strontium-doped lanthanum cobaltites (LSC), strontium-doped lanthanumferrite-cobaltites (LSCF), strontium-doped barium ferrite-cobaltites(BSCF), strontium-doped samarium cobaltites (SSC), and other perovskitesknown to those skilled in the art.

Oxygen electrode active materials may also include the so-calledRuddlesden-Popper (RP) phase materials having the general formulaA_(n−1)B_(n)O_(3n+1±δ), where A and B denote metal ions, O denotesoxygen, x indicates the level of A-site non-stoichiometry (excess ordeficiency), δ is indicative of oxygen non-stoichiometry, and n is aninteger. Relevant examples of RP phase materials include Ln₂NiO_(4+δ),where Ln is a lanthanide, A- or B-site doped Ln₂NiO_(4+δ), and other RPphases known to those skilled in the art. Ruddlesden-Popper phasematerials include double perovskites with a general formula(AA′)_(x)B₂O_(5+δ), where A, A′, and B are metal ions, O denotes oxygen,x indicates the level of A-site non-stoichiometry (excess or deficiency)and δ is indicative of oxygen non-stoichiometry. Examples of relevantdouble perovskites include materials such as LnBaCo₂O_(5+δ), where Ln isa lanthanide, and other double perovskites known to those skilled in theart.

In order to ensure good in-plane electrical conductivity over the cellactive area, contact layers are commonly deposited onto the electrodesof SOC. Oxygen-side contact layers typically comprise highly-conductiveoxide materials, such as the perovskites, double perovskites, or theRuddlesden-Popper phase materials listed above. In some cell designs,the electrode and contact layer functionalities are incorporated into asingle layer, i.e. the same layer acts both as the active electrode andthe contact layer.

In an SOC stack, a plurality of cells, each including a fuel electrode,an electrolyte, an oxygen electrode, and optionally contact layers, areconnected in series by interposing interconnection plates (orinterconnects) between each of the cells. The role of the interconnectsis to provide electrical contact from one cell to the next, and to aidin the distribution of gases across the cell. In order to reduceelectrical resistance arising from contact resistance between the cellsand the interconnects, it is of great importance that the contactingbetween the cells and the interconnects is of good quality, i.e.possessing low electrical resistance and excellent mechanical stabilityregardless of operating conditions.

Suitable materials for metallic interconnects need to be oxidationresistant against gases fed to both oxygen and fuel electrodes underelevated operation temperatures, and they must further exhibit a thermalexpansion coefficient (TEC) that matches the TEC of the ceramiccomponents of the cell. In view of these requirements, particularlyferritic alloys forming chromium oxide surface layers (e.g.chromia-forming ferritic steels) are used as materials for theinterconnect. Such alloys have a high chromium content (i.e. around15-26 wt. %) which forms a protective chromium oxide barrier layer onthe surface, protecting the interconnect against further oxidation.Examples of such high-chromium ferritic steels include, but are notlimited to AISI 441, AISI 444, AISI 430, AISI 446, Crofer 22H, Crofer22APU, ZMG G10, E-brite, Plansee ITM, etc.

During operation of an SOC stack, chromium species may diffuse from thechromium-containing metal interconnect materials into the adjacentoxygen electrode layers and thereby affect the catalyst performancedisadvantageously and thus limit the cell performance over time. Thisphenomenon is generally known as “chromium poisoning”. The chromiumpoisoning is due to the chromium in the metal interconnect beingtransported from the metal via gaseous chromium-containing oxides andoxy-hydroxides and to surface diffusion on the bridging metal oxidecomponents to the electrochemically active sites near to or on theoxygen side of the electrode, where they quickly deteriorate theelectrochemical activity to a considerable degree (J. Electrochem. Soc.,154 (4), 2007, pages A295-A306).

Coatings for SOC stack interconnects can be deposited with variousmethods. Most commonly these coatings are either deposited as a metal ora ceramic. Ceramic coating are most commonly based on Mn—Co spinelcompositions, whereas metallic coatings are most commonly based oncobalt. The main difference between metallic and ceramic coatingsbesides the deposition processes is that metallic coatings offer farbetter adhesion towards the ferritic steel interconnect. Adherence ofceramic coatings is based on van der Waals forces, whereas metalliccoating offers metallic bonds which in many cases supersede the bulkstrength of the ferritic steel material. The adhesion strength ofceramic coatings is furthermore dependent on a pre-oxidation stepcarried out in air in order to form a chromium oxide layer prior todeposition. The purpose of this pre-oxidation step is to add roughnesson the interconnects material to obtain a somewhat better adhesion ofthe as-deposited ceramic coating due to mechanical interlocking. Theceramic deposition process is furthermore not able to produce densecoatings, and the adhesion towards the interconnect material is known tobe problematic. For this reason, these coatings have the risk to spallupon heating and will therefore have inferior properties regardingprotection against chromium poisoning and high temperature oxidationcompared to metallic coatings.

Metallic coatings have the advantage that high adhesion strength towardsthe interconnect material can be obtained. Another advantage of metalliccoatings is that the metallic coating process is very easy to upscale.Furthermore, the metallic coating processes are already implemented on avery large scale (electroplating) and continuously developed by forexample the automotive industry. Therefore, electrodeposition ofmetallic coatings for interconnects use a far more developed processroute which is also advantageous from the perspective of productioncost.

In addition to chromium poisoning, another general problem leading todegradation or even to hard failure of SOC stacks is related to the(partial) loss of electrical contact between a cell and an interconnectin the stack. This (partial) loss of electrical contact is most likelyto occur during dynamic operation, for example when the SOC stack issubjected to load cycles or thermal cycles. These changes in operationwill inevitably create a thermal gradient across the SOC stack, whichcan have a negative influence on the mechanical contact betweeninterconnect and cell. If thermally induced stresses arising from thethermal expansion or contraction of the components exceed the bondingstrength between the interconnect and the cell, gaps can form atcell-interconnect contact points, effectively blocking electrontransport. In the most severe case, contact between cell andinterconnect is lost over a significant fraction of the cell activearea, leading to rapid increase in ohmic resistance through the stack,thus causing degradation.

It is, therefore, desirable to find a novel coating for SOCinterconnects, said coating being capable of ensuring contact points ofsufficient mechanical strength to the oxygen side of a solid oxide cell.

The present invention discloses an improved contact point betweeninterconnect and oxygen electrode material in a solid oxide cell stack.Generally, the main role of interconnect coatings is to slow down thevolatilization of chromium species from the interconnect (thus reducingthe risk of chromium poisoning) and to provide improved in-planeelectrical conductivity over the interconnect surface. It has nowsurprisingly been found that some coatings comprising certain elements,especially coatings comprising Cu, have the additional benefit ofimproving the mechanical strength and lowering the electrical resistanceof the contact between a coated metallic interconnect and either anoxygen-side contact layer (in case a contact layer is employed on theoxygen-side of the cell) or an oxygen electrode (in cell designs wherethe oxygen electrode acts both as the active electrode and contactlayer, as described above).

It has furthermore been found that these elements act as a sintering aidtowards some oxygen electrode materials and oxygen-side contact layermaterials, which results in an improved contact between the cobalt-basedinterconnect coating and the oxygen electrode material at hightemperatures. Here, the term ‘sintering aid’ refers to a functionaladditive or dopant that leads to a lowering of the sintering temperatureof a material. The addition of a sintering aid can reduce the sinteringtemperature of a material in a number of ways, such as by forming aliquid phase, thus promoting the densification through liquid-phasesintering, and by acting as a scavenging agent for impurities. A liquidphase can be formed either because the sintering aid lowers the meltingpoint of the bulk phase, because the sintering aid itself melts at thesintering temperature, or because the sintering aid forms a secondaryphase which melts at the sintering temperature.

During high-temperature treatment, a fraction of the Cu in the coatingdiffuses into the adjacent oxygen-side contact layer or oxygenelectrode. The mechanical strength (also referred to as pull-offstrength or adhesion strength or bonding strength) and electricalconductivity of a contact point formed in such a way is superiorcompared to copper-free coatings due to the lower sintering activityfound when copper is not present. The pull-off strength of a contactpoint can be evaluated for example by standardized dolly pull-off tests(e.g. ASTM D 4541 or ISO 4624) or modified three-point bending tests(e.g. Boccaccini et al., Materials Letters, 162 (2016), 250)).

So the present invention relates to a coated interconnect bonded to theoxygen electrode material of a solid oxide cell through the coating,which has obtained improved contact properties through sintering,thereby providing a strong bond between the interconnect and the oxygenelectrode material. More specifically, the invention concerns a contactpoint between a solid oxide cell and an interconnect of a solid oxidestack, said contact point comprising:

-   -   a ferritic stainless steel interconnect substrate covered by a        chromium oxide layer, which is coated by a coating comprising an        element that acts as a sintering aid, and    -   an oxygen electrode or an oxygen-side contact layer of a solid        oxide cell,        where the element functions as a sintering aid towards the        oxygen electrode or oxygen-side contact layer materials.

Further, the invention concerns a method for creating a contact pointwith a high mechanical strength between the coating on an interconnectand the oxygen electrode or the oxygen-side contact layer of a solidoxide cell (SOC), said method comprising the steps of:

-   -   providing a ferritic stainless steel interconnect substrate,    -   coating the oxygen side of the interconnect substrate with a        coating comprising an element that acts as a sintering aid,    -   providing a solid oxide cell, and    -   sintering the coated interconnect substrate and the solid oxide        cell by heat treatment in air,        where the element functions as a sintering aid towards the        oxygen electrode or oxygen-side contact layer materials.

The element that acts as a sintering aid is preferably Cu.

The coating on the metallic interconnect preferably comprises an oxideof Cu and Fe, an oxide of Cu and Ni, an oxide of Cu and Cu, an oxide ofCu, Co and Ni, or an oxide of Cu, Co, Ni and Fe.

Preferably, the oxygen electrode or oxygen-side contact layer materialcomprises a perovskite, a double perovskite, or a Ruddlesden-Popperphase material.

US 2003/0059335 A1 provides a high temperature material comprising achromium oxide forming an iron-based alloy containing a) 12-28 wt %chromium, b) 0.01 to 0.4 wt % La, c) 0.2 to 1.0 wt % Mn, d) 0.05 to 0.4wt % Ti, e) less than 0.2 wt % Si, f) less than 0.2 wt % Al with theproperty that at temperatures of 700° C. to 950° C. said hightemperature material is capable of forming at its surface a MnCr₂O₄spinel phase. According to the authors, the object of their invention isto provide a bi-polar plate for a high temperature fuel cell or forspark plugs. A disadvantage of said invention is that the interconnects(bipolar plates) produced this way will adhere poorly to the cells andthe contact points between the interconnect and cells will have a highcontact resistance.

US 2013/0230792 A1 discloses a coated interconnect for a solid oxidefuel cell including a substrate comprising iron and chromium and amanganese cobalt oxide spinel coating formed over an air side of theinterconnect substrate and a method of making and treating thereof. Adisadvantage of that invention is that the production of interconnectsby powder metallurgy and plasma spraying is very expensive and timeconsuming. Furthermore, the interconnect used in the above invention isnot ferritic stainless steel, but a CFY (Cr—Fe—Y) alloy, which isdesigned for solid oxide cells operating above 900° C.

A method of producing a protective coating on a Cr₂O₃ forming substrateis described in US 2006/0193971 A1. The method consists in applying amixture of CoO, MnO, and CuO onto a surface of the substrate alreadyhaving a layer of Cr₂O₃ and treating the substrate at 500-1000° C.,thereby converting the applied oxides to a gas-tight, chromium-freespinel coating on the substrate. However, as mentioned above, suchceramic coatings are disadvantageous compared to metallic coatings withrespect to the as-deposited adhesion strength towards the metallicinterconnect material.

This means that the described coating exhibits a low adhesion strength(van der Waals bonds) before it is heat treated to the resultingcoating. Therefore, there is a high risk of having spallation of thesetypes of coatings, thus creating contacting points having a lowmechanical integrity (weak interfaces) with respect to thermally inducedstresses.

U.S. Pat. No. 9,115,032 B2 discloses a method of densifying a lanthanidechromite ceramic or a mixture containing a lanthanide chromite ceramicby mixing the chromite ceramics with sintering aids and sintering themixture. The sintering aids comprise one or more spinel oxides, e.g.ZnMn₂O₄, MgMn₂O₄, MnMn₂O₄ and CoMn₂O₄. According to the authors,applications of such lanthanide ceramics include solid oxide fuel cells.

WO 2016/128721 A1, EP 2 267 826 A1, US 2005/0942349 A and EP 2 328 218A1 disclose various coatings containing oxides comprising Cu. Theobjective of each of the described inventions is to deposit coatingsthat enable enhanced corrosion protection and improvement of theelectrical conductivity, thereby lowering the ohmic resistance of theinterconnect. However, a coating comprising Cu can be considereddisadvantageous if such coating results in contact points with lowadhesion strength towards the oxygen electrode or the oxygen contactlayer of the solid oxide cell. During dynamic operation (load cycles,thermal cycles, changes in operating point) or due to interconnect creepduring long-term operation at a constant operating point, gaps can format cell/interconnect contact points, effectively blocking the electrontransport within the stack. This will lead to rapid increase in ohmicresistance throughout the stack, thus causing degradation and affectingthe robustness of the stack negatively.

A method to avoid inter-diffusion between metallic nickel andinterconnect is described in US 2009/0253020 A1. This is proposed to bedone by applying a cupriferous layer between the nickel-containing partof a fuel cell and the interconnect. It is furthermore proposed that theinterconnect undergoes a heat treatment to promote chromium oxide toform on the interconnect before applying the cupriferous layer. Theinvention described in US 2009/0253020 A1 relates to a known diffusionissue with Ni, causing austenite phase to form in the ferritic steelinterconnect, on the anode side of a fuel stack. Therefore, this doesnot relate to the present invention which has its focus on obtaining animproved contact point between oxygen electrode or oxygen contact layerand interconnect.

The present invention is described further in the examples which follow.In the examples, reference is made to the Figures, where

FIGS. 1a, 1b and 1c illustrate a contact point, a scanning electronmicroscopy (SEM) image of the contact point and the voltage drop acrossthe contact point, respectively, according to the prior art,

FIGS. 2a, 2b and 2c illustrate a contact point, a scanning electronmicroscopy (SEM) image of the contact point and the voltage drop acrossthe contact point, respectively, according to the present invention,

FIG. 3a shows the deposition of a third metallic layer on top of thestructure by ion exchange plating, further explained in FIGS. 3b and 3c, all according to the present invention, and

FIGS. 4a and 4b illustrate an EDX (energy-dispersive X-ray analysis)line scan (4 a) with point analysis (4 b), both according to the presentinvention.

EXAMPLE 1 (COMPARATIVE ART)

FIG. 1a presents a schematic drawing of a contact point 100 formed by acoated metallic interconnect and a solid oxide cell that can beconsidered prior art. The chromia forming ferritic stainless steelinterconnect 101 is covered by a chromia layer 102 and an oxide coating103 rich in Co, Mn, and Fe, but poor in Cr. The coated interconnect isin contact with the oxygen-side contact layer 104 of a solid oxide cell.FIG. 1b shows a scanning electron microscopy image of such a contactpoint. The adhesion strength of such a contact point is relatively low,as is evident from the micrograph, considering the interface betweencoating 103 and oxygen-side contact layer 104. The electrical propertiesof such a contact point were evaluated by exposing a structureconsisting of a porous LSCF disk with a diameter of 10 mm, a 0.3 mmthick square piece of a coated stainless steel interconnect with a sidelength of 20 mm, and another porous LSCF disk with a diameter of 10 mmto elevated temperatures in air. A direct current of 1 A was appliedthrough the structure, while a compressive loading of 3 MPa was appliedvia a load cell. Voltage drop through the structure is mostly governedby the resistance of the contact points, as the resistance of bulkinterconnect steel and bulk LSCF is much lower than contact pointresistance. According to FIG. 1c , voltage drop across such a contactpoint is approximately 5 mV at 900° C., 14.5 mV at 800° C., and 28 mV at750° C. After measurement, it is relatively easy to remove the LSCFdisks from the interconnect, indicating relatively low adhesion strengthof contact point.

EXAMPLE 2

FIG. 2a presents a schematic drawing of a contact point 200 formed by acoated metallic interconnect and a solid oxide cell according to thepresent invention. The chromia forming ferritic stainless steelinterconnect 101 is covered by a chromia layer 102 and an oxide coating203 rich in Co, Mn, Cu, and Fe, but poor in Cr. The coated interconnectis in contact with the oxygen-side contact layer 104 of a solid oxidecell. FIG. 2b shows a scanning electron microscopy image of such acontact point. The adhesion strength of such a contact point is expectedto be significantly higher than that of Example 1, as is evident fromthe micrograph. It is noteworthy that the oxide coating 203 haspartially diffused into the oxygen-side contact layer 104, and thatseveral particles of the oxygen side contact layer 104 are partially orcompletely encapsulated by the coating.

The electrical properties of such a contact point were evaluated usingthe same setup and under identical conditions as described in Example 1.According to FIG. 2c , voltage drop across such a contact point isapproximately 4 mV at 900° C., 10.5 mV at 800° C., and 20 mV at 750° C.After measurement, it is relatively much more difficult to remove theLSCF disks from the interconnect, indicating a relatively high adhesionstrength of the contact point compared to Example 1.

EXAMPLE 3

A metallic coating on the surface of a ferritic stainless steelinterconnect substrate 101 is formed by coating the oxygen side of theinterconnect substrate first with a strike layer of Co or Ni 301 byelectrodeposition, followed by electrodeposition of an additional layer302 consisting of Co on top of the strike layer 301. A third metalliclayer of Cu 303 is deposited by ion exchange plating on top of thestructure comprising the interconnect substrate 101 and the coatinglayers 301 and 302 (FIG. 3a ). The thickness of the Cu layer 303 isapproximately 100-200 nm. To form the contact point 200, thus formedcoated interconnect 304 is taken into contact with the oxygen-sidecontact layer 104 of a solid oxide cell at a temperature exceeding 800°C. This step is explained as A in FIG. 3b and FIG. 3c . At thistemperature, the metallic coatings 301, 302 and 303 are oxidized,forming an oxide coating 203 rich in Co, Mn, Cu, and Fe in the case of aCo strike layer (FIG. 3b ), and 204 rich in Co, Mn, Cu, Fe with smallamounts of Ni in the case of a Ni strike layer (FIG. 3c ). Both formedoxide coatings 203, 204 are thus poor in Cr. Simultaneously, a chromialayer 102 is formed between the interconnect substrate 101 and the oxidecoatings 203 and 204. Also simultaneously, a fraction of the Cu in theoxide coatings 203 or 204 diffuses into the the oxygen-side contactlayer 104 of a solid oxide cell, acting as a sintering aid. Hereby, thecontact point 200 (FIG. 2a ) is formed in the case where the oxidecoating is 203. In FIGS. 4a and 4b , an EDX (energy-dispersive X-rayanalysis) line scan with point analysis across the interface of theoxide coating 203 and the oxygen-side contact layer 104 is shown,indicating that a fraction of the Cu from the oxide coating 203 hasdiffused into the oxygen-side contact layer 104.

The invention claimed is:
 1. A contact point between a solid oxide celland an interconnect of a solid oxide stack, said contact pointcomprising: a ferritic stainless steel interconnect substrate covered bya chromium oxide layer and at least one metallic layer comprising Co orNi, the at least one metallic layer being coated by a coating comprisingCu having a thickness of approximately 100-200 nm, and an oxygenelectrode or an oxygen-side contact layer of a solid oxide cell, whereinthe Cu in the coating functions as a sintering aid towards the oxygenelectrode or oxygen-side contact layer materials, and a fraction of theCu in the coating diffuses into the oxygen electrode or oxygen-sidecontact layer of the solid oxide cell, thereby increasing the adhesionstrength and lowering the electrical resistance of the contact pointbetween the coated interconnect substrate and the oxygen electrode oroxygen-side contact layer of the solid oxide cell, such that the voltagedrop across the contact point is less than 25 mV, when measured in airat 750° C., under a dc current density of 1.27 A/cm², under acompressive load of 3 MPa.
 2. A contact point according to claim 1,wherein the coating on the metallic interconnect comprises an oxide ofCu and Fe, an oxide of Cu and Ni, an oxide of Cu and Cu, or an oxide ofCu, Co and Ni, or an oxide of Cu, Co, Ni and Fe.
 3. A contact pointaccording to claim 1, wherein the oxygen electrode or oxygen-sidecontact layer material comprises a perovskite, a double perovskite, or aRuddlesden-Popper phase material.
 4. A contact point according to claim1, wherein the adhesion strength of the contact point is of the sameorder of magnitude as the adhesion strength between the electrolyte andthe barrier layer of the solid oxide cell.
 5. A contact point accordingto claim 1, wherein the operating temperature of the solid oxide cellstack is between 500° C. and 900° C.
 6. A method for creating a contactpoint between a coating on an interconnect and an oxygen electrode oroxygen-side contact layer of a solid oxide cell (SOC), comprising thesteps of: providing a ferritic stainless steel interconnect substrate,depositing at least one layer of Co or Ni on an oxygen side of theinterconnect; coating the layer of Co or Ni on the oxygen side of theinterconnect with a coating comprising Cu having a thickness ofapproximately 100-200 nm, providing a solid oxide cell, and sinteringthe coated interconnect substrate and the solid oxide cell by heattreatment in air at a temperature exceeding 800° C., where the Cu in thecoating functions as a sintering aid towards the oxygen electrode oroxygen-side contact layer materials, and a fraction of the Cu in thecoating diffuses into the oxygen electrode or oxygen-side contact layerof the solid oxide cell, thereby increasing the adhesion strength andlowering the electrical resistance of the contact point between thecoated interconnect substrate and the oxygen electrode or oxygen-sidecontact layer of the solid oxide cell, such that the voltage drop acrossthe contact point is less than 25 mV, when measured in air at 750° C.,under a dc current density of 1.27 A/cm², under a compressive load of 3MPa.